if you try 5=5 you get an 'invalid lvalue in assignment'-like error, is it what you mean? it happens when the left side of an assignement is not...assignable (i.e. a variable).
I don't see any situation in which you would get the equivalent for an rvalue, as it would rather result in a syntax error...
What was the point of the question? any context?

A (modifiable) lvalue is required for assignment. If you have this = that; then this has to be an lvalue. There are no such things as rvalues in C, although sometimes people use it to mean "something that could be on the right-hand side of a assignment".

if you try 5=5 you get an 'invalid lvalue in assignment'-like error, is it what you mean? it happens when the left side of an assignement is not...assignable (i.e. a variable).
I don't see any situation in which you would get the equivalent for an rvalue, as it would rather result in a syntax error...
What was the point of the question? any context?